Electron beam device having means
bending beam into critical curva-
ture, thereby maintaining trans-
verse coherency of electrons in
beam cross-section



June 1966 J. w. KLUVER ELECTRON BEAM DEVICE HAVING MEANS BENDING BEAM INTO CRITICAL CURVATURE, THEREBY MAINTAINING TRANSVERSE COHERENGY OF ELEGTRONS IN BEAM CROSS-SECTION Filed Dec. 10, 1962 I I I h l INVEMZ'OR J. n. KLUVER A 7' TORNE V United States Patent C 3,258,638 ELECTRON BEAM DEVICE HAVING MEANS BENDING BEAM INTO CRITICAL CURVA- TURE, THEREBY MAINTAINING TRANS- VERSE COHERENCY F ELECTRONS IN BEAM CROSS-SECTION Johan W. Kliiver, Murray Hill, N J., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Filed Dec. 10, 1962, Ser. No. 243,391 3 Claims. (Cl. SIS-3.5)

This invention relates to electron beam devices and, more particularly, to M-type devices.

The term M-type device refers to an electron tube which uses mutually perpendicular electric and magnetic fields for constraining electron flow, as distinguished from 0- type devices which use longitudinal magnetic fields for this purpose. The most common M-type device is the conventional magnetron which comprises a circular cathode surrounded by a concentric anode. The most attractive features of the M-type device is its efliciency and capacity for high power operation. Efforts to use it as an amplifier, however, have been disappointing, as compared to O-type devices, because of characteristic problems of instability and noise. It has been determined that these problems result primarily from the incoherent character of the electron stream or electron cloud as it flows between the cathode and anode.

Accordingly, considerable effort has been expended in developing electron guns for forming and projecting crossed-field focused electron beams that are coherent in the sense that they have definable boundaries. In M-type devices using coherent beams, a drift region is typically defined between a linear anode and a linear negativelybiased electrode, referred to as a sole plate. The most common type of gun for injecting electrons into the drift region is the Charles gun which comprises a cathode at one end of the drift region for projecting electrons at a 90 degree angle with respect to the tube axis. Focusing fields force the electrons into the drift region in a direction parallel with the tube axis. In the drift region the electric and magnetic fields maintain a fairly coherent beam crosssection and prevent the electrons from impinging on the anode and sole plate as they travel toward a collector at the opposite end.

The linear anode of this device can be conveniently defined by a slow-wave structure which amplifies an electromagnetic wave by transmitting it in synchronism with the beam according to the principles of traveling wave interaction. M-type devices are also promising as electron beam parametric amplifiers. In this type of device, the beam is modulated with radio-frequency energy, the modulations are amplified by a pump frequency, and the amplified modulations are then removed. Still another use derives from the extremely low electron beam velocities that are possible in M-type devices. A signal can be substantially delayed by modulating the beam, letting the beam drift at a low velocity, and then demodulating it to recover the signal.

I have found that all of these operations are degraded by electron velocity variations over the transverse beam cross-section. The beam can be visualized as comprising a series of beam lamina each of which flows toward the collector at a different velocity. As will be explained more fully hereafter, these velocity variations reduce the attainable power and efiiciency of M-type devices in general, they increase the noise in M-type parametric an1 plifiers, and they cause signal loss in M-type delay lines.

Accordingly, it is an object of this invention to eliminate or reduce substantially the deleterious effects of electron velocity variations over the cross-sections of electron beams of M-type devices.

This and other objects of the invention are attained in an illustrative embodiment thereof which serves as an electronic delay line. The device includes an electron gun for forming and projecting a beam of electrons toward a collector at a relatively low velocity. The beam is focused by crossed electric and magnetic fields, both of which are perpendicular to the beam path. An extended anode and sole plate on opposite sides of the beam path produce the electric focusing field and define a drift region. Signal energy is introduced to the beam by an input coupler at one end of the drift region and is extracted from the beam by an output coupler at the other end. The

time taken by the beam to transport the signal from the input coupler to the output coupler is readily determinable and represents a substantial and useful delay.

It has been found that in an electron beam of this type the electrons nearest the anode flow the fastest, with other electron velocities decreasing approximately linearly in the direction of the sole plate. It has further been found that the rate of change of electron velocity over a transverse cross-section varies inversely with the cyclotron frequency of the electrons and in direct proportion to the square of the plasma frequency of the electrons.

It is a feature of this invention that the electron beam path be curved with the anode defining the outer boundary of the drift region and the sole plate the inner boundary. In accordance with discoveries I have made, the radius of curvature R of the beam path is defined by the relation where o is the plasma frequency of the electron, b is the beam thickness, a is the separation between the anode and the beam path, 0 is the separation between the sole plate and the beam path, log is the natural logarithm, and V is the voltage on the anode with respect to the sole plate. By this device, the electrons nearer the anode are forced to follow longer paths in proportion to their faster velocities so that all of the electrons are maintained in synchronism as they travel from the electron gun to the collector. This compensates for the transverse velocity variations and eliminates the malfunctioning that would ordinarily result from them.

These and other objects and features of the invention will be more clearly understood from a consideration of the following detailed description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a sectional view of an electronic delay device employing the principles of the invention;

FIG. 2 is a section taken along lines 2-2 of FIG. 1; and

FIG. 3 illustrates schematically a small section of the electron beam of the device of FIG. 1.

FIGS. 1 and 2 show a delay device 11 comprising an electron gun 12 for forming and projecting a low-velocity electron beam along a path 13 toward a collector 14. The electron gun is shown as comprising a cathode 15, having an electron-emissive coating and an accelerating electrode 16. An extended sole plate 18 and an anode 19 are located on opposite sides of the beam path. The anode is maintained ata positive direct-current potential with respect to the sole plate to produce an electrostatic focusing field transverse to the beam path. A magnetic field transverse to both the electric field and the beam path is formed by a magnet 20 shown in FIG. 2. The magnetic field exerts a downward force on the electrons to counterbalance the upward force of the electric field. The cathode may be maintained at the same potential as the sole plate or at a different potential. It should be noted, however, that the sole plate does not emit or collect electrons; the electron beam is coherent in the sense that it has definable boundaries.

Input electromagnetic signal energy is transferred to the beam by an input resonator 21. As explained in my co-pending application, Serial No. 224,726, filed September 19, 1962, and assigned to the Bell Telephone Laboratories, Incorporated, wave energy can be transmitted by a very low-velocity M-type electron beam if the beam is modulated in the fast cyclotron mode. Accordingly, resonator 21 is resonant at the cyclotron frequency as is required for fast cyclotron mode modulation. The signal energy is then transmitted by the low-velocity beam to an output resonator 22 where it is removed. The output resonator is substantially identical to the input resonator. The time period of signal transmission by the low-velocity electron beam constitutes an appreciable predetermined delay of the signal which can be useful in a number of known systems. The anode 19 is a ribbonshaped conductor and is shown as being segmented. This can be done to permit optimum voltages to be used in the input and output couplers, 21 and 22, or alternatively, it can be a continuous strip. Input and output signal waves are conveniently transmitted by coaxial cables as shown by the arrows.

Normally, a coherent M-type electron beam of the type shown in the figure is projected along a linear path between a planar sole plate and anode. This type of beam can be considered as being laminated by imaginary planes parallel with the sole plate and anode. It can be shown that the beam lamina nearest the anode travels faster than the mean beam velocity, the one nearest the sole plate travels slower, and the intervening lamina travel at intermediate velocities. This velocity variation causes certain spurious beam waves, known as synchronous waves, to couple actively and become amplified according to known principles of electron interaction. These synchronous waves are defined by electron displacement modulations so that their amplification results in spurious beam expansion. This phenomenon has been observed by workers in the art and is known as the diocotron effect. It is known to limit the operational range, power, and efficiency of virtually all M-type devices. In a delay line of the type shown in the drawing it is particularly disadvantageous because it will ordinarily cause signal loss at very low beam velocities.

These deleterious effects are obviated in the device of FIG. 1 by forcing the beam to follow an arcuate path of predetermined radius. It can be shown that if the spacecharge density is assumed to be constant, a physically realizable electron flow is obtained if all of the electrons have the same average angular velocity 01,-, which is given by the relation where r is the radial coordinate and 1 is the charge-tomass ratio of an electron. The field described in Equation 3 is formed inside the beam if the voltage V on the anode, with respect to the sole plate, is given by the relation Rblog +Rb =V where b is the beam thickness, a is the separation between the anode and the beam path, 0 is the separation between the sole plate and the beam path, log is the natural logarithm, and V is the voltage on the anode with respect to the sole plate. Dimensions a, b, and c are illustrated in FIG. 3. It can be seen that R is the radius of curvature of the center of the beam, while the radius of the edges of the beam are Rib/2, the radius of the anode is R-i-a, and the radius of the sole plate is Rc. It follows that beam path, the anode, and the sole plate are defined by arcs of concentric circles.

When w w which is a practical situation, Equation 2 reduces to 2 o The tangential velocity v of electrons at radius R is given by From the Equations 5 and 6, and using the well-known definitions of u and w the following relation is derived:

2e BAv I (7) As mentioned above, the properly curved. beam path is defined by using an anode curvature of radius (R-l-a), a sole plate curvature of radius (R-c), and the proper values of A, V, I, B and (a-l-c) to fulfill the stated requirements. With these requirements met, the electrons will curve to remain at right angles with the electric field. Moreover, in accordance with the invention, all of the electrons will move at the same angular velocity and will remain in spatial synchronism. This improvement eliminates spurious synchronous wave coupling and its attendant disadvantages. Specifically, the precise curvature of the beam path permits the electron beam to transport energy from input coupler 21 to output coupler 22 at a very low velocity with a minimum of attenuation and distortion.

The invention was shown as being used in a delay device only for illustrative purposes. It is also useful in M-type traveling wave amplifiers, oscillators, parametric amplifiers, and in any other tube that uses an M-type coherent beam. Other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is: ll. An electron discharge device comprising: means for forming and projecting a stream of electrons along a path; means for establishing a magnetic field transverse to the path; means comprising a curved anode and a curved sole plate for establishing an electric field that is transverse to the path and the magnetic field; the electric and magnetic fields together comprising means for maintaining a radius of curvature R of the path Which is substantially determined by the relation where B is the magnetic field flux density, A is the crosssectional beam area, I is the beam current, s is the dielectric constant for a vacuum, V is the voltage on the anode with respect to the sole plate, a is the separation between the beam path and the anode and c is the separation between the beam path and the sole plate.

2. An electron discharge device comprising: means for forming and projecting a coherent beam of electrons; means for maintaining beam coherency comprising a curved anode and a curved sole plate for producing an electric field therebetween; the said path being arcuate and having a radius of curvature R substantially determined by the relation 2e 2 R+4 ge 6 where w is the plasma frequency of the electrons, b is the beam thickness, a is the separation between the anode and the beam path, 0 is the separation between the sole plate and the beam path, log is the natural logarithm, and V is the voltage on the anode with respect to the sole plate;

the radius of curvature of the anode being substantially equal to (R-l-a); the radius of curvature of the sole plate being substantially equal to (R-c); and means for producing a magnetic field which is perpendicular to the electric field and to the beam path. 3. An electron discharge device comprising: means for forming and projecting a coherent beam of electrons; means for modulating the beam; means for demodulating the beam; means defining an arcuate beam path comprising an arcuate sole plate and an adjacent arcuate anode; the radius of curvature of the beam path being substantially determined by the relation 2 {wart/13%} References Cited by the Examiner UNITED STATES PATENTS 2,951,173 7/1960 Mourier et al 3l539.3 X

HERMAN KARL SAALBACH, Primary Examiner.

S. CHATMON, JR., Assistant Examiner. 

1. AN ELECTRON DISCHARGE DEVICE COMPRISING: MEANS FOR FORMING AND PROJECTING A STREAM OF ELECTRONS ALONG A PATH; MEANS FOR ESTABLISHING A MAGNETIC FLUID TRANSVERSE TO THE PATH; MEANS COMPRISING A CURVED ANODE AND CURVED SOLE PLATE FOR ESTABLISHING AN ELECTRIC FIELD THAT IS TRANSVERSE TO THE PATH AND THE MAGNETIC FIELD; THE ELECTRIC AND MAGNETIC FIELDS TOGETHER COMPRISING MEANS FOR MAINTAINING A RADIUS OF CURVATURE R OF THE PATH WHICH IS SUBSTANTIALLY DETERMINED BY THE RELATION 